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Post by OldGit on Nov 13, 2023 21:47:54 GMT 1
Well, here's a strange one! The other diag tech at work bought a trade-in 14 plate C class a few months ago - bit of a chain rattle & the rocker cover leaking were pretty much the only issues. On Saturday, he replaced the timing chain & rocker cover gasket, cleaned up the oil and went for a triumphant start - except it struggled to start, hunted a bit, faded out (like fuel or air starvation) and refused to fire up, but made a few weedy attempts at 'almost firing but not quite'.
The combined brains came up with the following checks in no particular order
Timing - OK Compression - OK CMP/CKP signals - OK DTC's - stored 'circuit 87 open' fault, indicative of the relay dropping out during cranking due to low voltage Fuel pressure low & high - OK, 5bar / 300bar
Throttle body & EGR positions - OK Injectors 'airlocked' - OK, fuel coming out of leak-off ports on all injectors.
Finally we rigged an injector onto the rail and cranked - nothing.... found another OM651, put the injector on that rail, still nothing - voltages at injectors on both engines at cranking near-identical. It looks like all four injectors have failed, one of the techs today mentioned 'Oh yer, I've had that before if you don't bleed up to the injectors and crank more than five times' - something I've never heard of or known across many brands.... Personally I think it's something to do with low voltage during cranking (he didn't have a charger on the battery after repair) and the fuel rail being empty of fuel / full of air, but it's something I've never come across before...
Any ideas? he's ordered a set of injectors (£1k at staff prices) but we're both looking for the likely cause, because it just doesn't make sense - none of the injectors were dropped, disconnected when powered etc. etc.
The worst of it was, he had to suffer being given a lift home in a Land Rover....
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Post by Noberator on Nov 13, 2023 21:51:30 GMT 1
How old is the battery and what's the voltage when it's stood?  |
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Post by OldGit on Nov 13, 2023 22:05:13 GMT 1
The battery is ~18 months old, the BMS wasn't reporting any concerns but it's possible the charge level was less than optimal - I don't want to come across as a know-it-all, I wouldn't have tried starting the negine without a charger / BSU connected personally...
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Post by valhalla on Nov 13, 2023 22:31:28 GMT 1
This is a new one for me as well - doesn't make sense IMHO.
The only thing I could correlate to this would be if the injector-control maps have got some sort of residual battery-voltage correction data left-over from the (discharged battery) cranking previously. The point of this is less obvious for piezo units, but such corrections do still happen on later drivers. It used to be much more important on earlier solenoid injector units.
Might be worth having a peek at the injector durations during cranking - just see if MB have done something silly here. If you have a car to compare-with, all the better, otherwise I could try to dig-out some generic data from my work with LR.
Has anyone tried a battery-reset and full power-down on the car yet?
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Post by OldGit on Nov 13, 2023 22:51:30 GMT 1
The other thing that occurred to me was the ECM has some pretty hefty capacitors to carry it through cranking voltage dips, I wonder if these could / would cause the issue if they were losing charge then suddenly got an input boost? We did a battery disconnect reset, an ECM reset, reloaded the string coding, crash input reset, checked injector coding, reset injector coding, cleared airpath adaption values.... I wasn't there yesterday but he did scope the injector pulses, reckoned they were very similar to the known-good vehicle with the LP fuel pipe clamped off to stop it firing up. It's like the piezo's have 'burnt out'.... especially with no output when plumbed into the 'known good' fuel system.
At this stage, it's really a post-mortem - just to understand why it happened, I think he's going to raise a tech case with M-B, there's certainly no warnings in WIS about bleeding the fuel system before reconnecting the injectors and there's been plenty of CR diesel engines, M-B and others, that have been cranked over with lower fuel rail pressure for longer with no ill effects...
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remmington
Apprentice
Owns Spark Eroder
Posts: 4,971
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Post by remmington on Nov 14, 2023 7:23:32 GMT 1
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Post by OldGit on Nov 14, 2023 8:15:33 GMT 1
Piezo injectors - but with return pipes, sometimes the internet isn't entirely correct....
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Post by Rhubarb on Nov 14, 2023 8:24:22 GMT 1
Does it run on a sniff?
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Post by OldGit on Nov 14, 2023 8:29:51 GMT 1
Yep - mechanically it's fine - airpath too.
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Post by Rhubarb on Nov 14, 2023 8:54:22 GMT 1
Yep - mechanically it's fine - airpath too. I've known one of these to have the same symptoms after a rebuild. Timing was out. |
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remmington
Apprentice
Owns Spark Eroder
Posts: 4,971
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Post by remmington on Nov 14, 2023 11:44:33 GMT 1
Piezo injectors - but with return pipes, sometimes the internet isn't entirely correct.... I not seen any of these without return pipes myself.
I have broken loads of these sprinter plastic return pipes taking them off thou.
Plus seen loads melted with the "black death"... |
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Post by OldGit on Nov 14, 2023 16:20:22 GMT 1
Yep - mechanically it's fine - airpath too. I've known one of these to have the same symptoms after a rebuild. Timing was out. We rechecked the timing, cam to cam & cam to crank, compression is good too. Even if the timing was out, there'd still be fuel injection happening - there's no output from any injector... Very odd. |
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Post by OldGit on Nov 14, 2023 16:22:41 GMT 1
Piezo injectors - but with return pipes, sometimes the internet isn't entirely correct.... I not seen any of these without return pipes myself.
I have broken loads of these sprinter plastic return pipes taking them off thou.
Plus seen loads melted with the "black death"...The leak-off pipes are meant to be changed along with the copper seats and hold down bolts every time the injectors are removed. |
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Post by givusaclue on Nov 14, 2023 18:40:30 GMT 1
never personally come across that fault, done loads of these.
what's the hp reading at the rail?
have you got access to a guage set that can read actual physical pressure at the rail, not by using the existing vehicle frp sensor?
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Post by OldGit on Nov 14, 2023 19:20:24 GMT 1
That's the odd thing, as a workshop we've done 'loads' - I've done fifteen in the last year, never had an injector issue. FRP measured at 295bar, indicated at 300bar, no air in LP side, which was a measured 5bar. New injectors arriving in the morning!
Still, at least the chain won't rattle at startup....
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